Portable (also commonly known as "cordless") electrical tools and other devices, appliances, and equipment (such as video cameras and the like) require an adequate supply of power from battery packs that are typically received in a portion of the tool housing. However, because battery packs are inherently heavy, as explained in application Ser. No. 08/740,695 filed on Nov. 1, 1996 by the assignee of the present application and incorporated herein by reference, a complaint frequently expressed by users of cordless devices is that they are too heavy. To reduce forearm fatigue and other user soreness typically associated with cordless devices (and particularly with heavy cordless power tools), it is well known to locate the batteries required to power the tool in a garment or belt worn by the user.
In this regard, U.S. Pat. No. 5,211,321 issued on May 18, 1993, to N. Rodriguez, entitled "Battery And Equipment Vest," discloses a garment configured as a vest to receive several battery cells to provide power to video recording equipment. The vest includes several storage pockets placed around the waist of the user to receive the cells. Alternatively, the vest may be associated with a belt having pockets configured to house the cells. The garment may also include a device to recharge the cells when required, thereby providing a mobile and convenient power source. The battery and equipment vest has adjustable shoulder straps and adjustable waist to ensure a comfortable fit for multiple users while orderly maintaining electrical interconnections.
In other cases, as in U.S. Pat. No. 4,748,344 issued in May 31, 1988 to Sing, the weight of the battery is distributed along a belt disposed around the waist of the user. Sing also discloses that the power supply belt can include a selector switch so that the power source can be used with a variety of electrical implements having different operating voltages. In the case of U.S. Pat. No. 3,919,615, issued on Nov. 11, 1975, to R. Niecke, the belt also includes an inverter to allow its use with AC powered tools.
In all of these cases, however, the resistance of the cord connecting the power source, i.e., the garment or the belt, to the tool causes line losses which manifest themselves by a voltage drop at the tool particularly when the tool operates under significant load conditions. As a result, the tool becomes powered at a voltage that is less than its rated voltage, thereby reducing the amount of work that can be produced by the tool when it is most needed. Connections between the power source and the cord and between the cord and the tool also frequently create additional losses thereby compounding the problem. Specifically, line and connection losses generally reduce the effectiveness of the tool, both in terms of the work that can be produced by the tool and the amount of time the tool can be used without having to recharge the battery pack. This is because a portion of the energy stored in the battery pack is wasted in line and connection losses instead of being usable by the tool to be converted into work.
To compensate for these detrimental voltage losses and attempt to preserve the designed effectiveness of the tool, prior art power sources often deliver a voltage that is higher than the rated voltage of the tool with which the power source is associated. The drawback of this approach, however, is that under limited load conditions (e.g., when a relatively small amount of work is required of the tool), the current drawn by the tool is correspondingly small thereby causing the tool to operate at a voltage that is greater than its rated voltage. As is well known in the art, such over-voltage conditions may damage certain features of the tool such as for example the tool variable speed trigger circuit.
Accordingly, although these prior art portable power supplies have advantageously shifted the weight of the battery packs from the tool to the body of the user thereby rendering the use of cordless tools more comfortable to the user, these power supplies still suffer from the shortcomings generally identified above. Thus, it has become apparent to the inventors of the present invention that it is desirable to find alternate ways to compensate for line losses between the power source and the tool. In other words, it seems desirable to develop ways to more effectively power cordless tools over a broad range of operating conditions without unduly increasing the cost of portable power sources associated with these tools.